1. Field of the Invention
The present invention generally relates to nickel-base alloys. More particularly, this invention relates to a castable and weldable nickel-base superalloy that exhibits desirable properties suitable for gas turbine engine applications.
2. Description of the Related Art
The superalloy IN-738 and its low-carbon version (IN-738LC) have a number of desirable properties for gas turbine engine applications, such as inner shrouds, latter-stage buckets (blades), and nozzles (vanes) in the turbine section of an industrial gas turbine. The composition of IN-738 can vary slightly among producers, with one publication listing the IN-738 composition, by weight, as 15.7-16.3% chromium, 8.0-9.0% cobalt, 1.5-2.0% molybdenum, 2.4-2.8% tungsten, 1.5-2.0% tantalum, 0.6-1.1% columbium (niobium), 3.2-3.7% aluminum, 3.2-3.7% titanium (Al+Ti=6.5-7.2%), 0.05-0.15% zirconium, 0.005-0.015% boron, 0.15-0.20% carbon, the balance nickel and impurities (e.g., iron, manganese, silicon and sulfur). IN-738LC differs in its boron, zirconium and carbon contents, with suitable ranges for these constituents being, by weight, 0.007-0.012% boron, 0.03-0.08% zirconium, and 0.09-0.13% carbon.
As with the formulation of other superalloys, the composition of IN-738 is characterized by controlled concentrations of certain critical alloying elements to achieve a desired mix of properties. For use in gas turbine applications, such properties include high temperature creep strength, oxidation and corrosion resistance, resistance to low cycle fatigue, castability and weldability. If attempting to optimize any one of the desired properties of a superalloy, other properties are often adversely affected. A particular example is weldability and creep resistance, both of which are of great importance for gas turbine engine buckets. However, greater creep resistance results in an alloy that is more difficult to weld, which is necessary to allow for repairs by welding.
While IN-738 has performed well in certain applications within gas turbine engines, alternatives would be desirable. Of current interest is the reduction in tantalum used in view of its high cost. Though tantalum nominally constitutes only about 1.8 weight percent of IN-738, its reduction or elimination would have a substantial impact on product cost in view of the tonnage of alloy used.